The present invention relates generally to devices for fixation of parts of a fractured bone and more specifically, to bone plates and systems for stabilization and/or compression of parts of a fractured bone.
Bone plates may generally be utilized to carry out two different types of osteosynthesis, namely xe2x80x9crigid osteosynthesisxe2x80x9d and xe2x80x9cflexible osteosynthesis.xe2x80x9d Rigid osteosynthesis is used for medical care of joint fractures, simple shaft fractures (where nailing is impossible) as well as for osteotomies. Aside from the possibility of anatomical repositioning, the bone itself supports and stabilizes the osteosynthesis, which allows for the possibility of putting stress on the extremity earlier and without pain. Additional advantages of the medical care of stable fractures can be observed when the blood circulation in the bone is greatly diminished due to trauma. For treating xe2x80x9cnonunionsxe2x80x9d or in the case of existing infection, the fracture must be kept stable in order to make bone healing possible and so as not to irritate the infection further by instability of the fracture gap.
Flexible osteosynthesis, also known as xe2x80x9cbiological osteosynthesis,xe2x80x9d may be desirable in the medical treatment of comminuted fractures in the shaft region of tubular bones. In the case of these fractures, it is an objective to maintain the proper length of the bone and to fix the bone ends joints) in their proper anatomic positions with respect to one another. With flexible osteosynthesis, the fracture zone is not directly affixed or manipulated, and consequently, the blood circulation in this area is not inhibited. Bone plates designed for flexible osteosynthesis thus operate similarly to a locking, intramedullary nail, which is anchored only in the metaphyses.
Since fractures cannot always be treated with one type of osteosynthesis, surgeons must frequently compromise because a bone plate, which allows him to combine the two types of osteosynthesis discussed above, is not available. Such a combination would be beneficial, for example, when a joint fracture can be compressed with traction screws through the bone plate and the whole of the joint may be connected to the diaphysis over an internal fixative with angularly stable screws. Another illustrative application concerns porotic bones, where a bone plate with axially and angularly stable screws can be anchored in the metaphysial fragment, with a stable plate-affixation being undertaken in the diaphyseal range with the assistance of a plate traction screw through the fracture. A primary fracture stabilization can be achieved by this type of procedure.
This situation has led to the development and marketing of bone implants for both types of osteosynthesis. The two types of implant group, however, are designed specifically for their respective method. Thus, the disadvantages of these two systems lies in the difficulty in combining them.
Thus, a need exists for improved bone plates that provide for both rigid and flexible osteosynthesis.
The present invention is directed to a bone plate that is adapted to be used for both rigid and flexible osteosynthesis, without compromising the plates ability to be used for either type of osteosynthesis. Accordingly, the bone plate of the present invention may be used as a compression plate or as an internal fixative.
According to one embodiment of the invention, the bone plate includes an upper surface, a bone contacting surface, and a plurality of holes extending through the upper and bone contacting surfaces. At least one of the holes is elongated in a direction substantially aligned with a longitudinal axis of the plate, and includes a threaded portion and a non-threaded portion. The threaded portion may extend over a range of greater than about 180xc2x0 with respect to a central axis of the hole. The threaded portion of the hole is dimensioned and configured to engage a threaded head of a bone screw, and fix the bone screw at a predetermined angle with respect to the bone plate. Preferably, the threaded portion extends through the full thickness of the bone plate, i.e., from the upper surface to the bone contacting surface, thus maximizing the stability of the bone screw to bone plate interface.
With the threaded screw head fixed in the threaded portion of the elongated hole, the bone plate may be used as an internal fixative. Use in this configuration, however, creates high stresses at the interface of the bone plate and screw head because the plate is not forced against the bone, and therefore, the bone fracture is fixed primarily by friction between the plate and the bone. This increase in stress is taken into account by the threaded portion of the hole extending over a range of at least about 180xc2x0 with respect to a central axis of the hole, and thereby enclosing the screw head in at least this angular range. This feature of the bone plate is especially advantageous where thin bone plates are involved. Preferably, the threaded portion is disposed on one of the two longitudinal ends of the hole. This positioning allows for the threaded portion to extend over a larger angular range. For example, the threaded portion may extend over a range of between about 190xc2x0 and about 280xc2x0, and preferably over a range of between about 200xc2x0 to 250xc2x0, thus maximizing the strength of the bone screw to bone plate interface.
According to another embodiment of the present invention, at least one of the holes may include a threaded portion that is angled or tapered with respect to a central axis of the hole. More specifically, the threaded portion may conically taper inward towards the bone-contacting surface of the bone plate. A bone screw to be rigidly fixed to the bone plate may include a threaded screw head that is tapered to substantially match the tapered shape of the threaded portion of the hole. Thus, the bone screw may be rigidly fixed to the bone plate by engagement between the matching conical threads. This method of attachment is especially advantageous when self-drilling screws are to be used since, due to the conical shape of the matching threads, the screw may be inserted into the bone independently of the plate. More specifically, the screw head becomes rigidly clamped to the plate only as the threaded screw head penetrates the threaded portion of the hole. Despite any initial misalignment between the threads on the screw head (the position of which are initially dictated by the orientation of the bone screw in the bone) and the threads on the bone plate, the conical shape of the mating threads ensures that the threads on the screw head will ultimately align with the threaded portion of the hole. When the conical thread screw head is tightened into the threaded portion of the hole, the screw head creates radial forces in the plate hole. Thus, the bone plate must be dimensioned and configured to withstand these high radial forces, e.g., to withstand flexing of the walls of the screw holes in the bone plate.
The threaded portion preferably tapers at a cone angle of between about 5xc2x0 and about 20xc2x0. Preferably, the thread tapers at a cone angle of about 10xc2x0.
In the case where the threaded portion of the hole is tapered, as discussed above, the threaded portion may extend through a different angle when measured at the upper surface than when measured at the bone-contacting surface. For example, when measured at the upper surface, the threaded portion may extend through a first angle of between about 200xc2x0 and about 270xc2x0, while when measured at the bone-contacting surface, the threaded portion may extend through a second angle of between about 180xc2x0 and about 230xc2x0.
According to another aspect of the present invention, at least one of the holes may be dimensioned and configured to receive a ball shaped head of a bone screw and provide for compression of two fractured bone fragments. For example, according to one embodiment, the non-threaded portion of the elongated hole, discussed above, may include a concave, substantially spherical recess at the upper surface. The recess may be dimensioned and configured to accommodate the spherical head of a conventional bone screw. Such an arrangement may be especially useful when the bone screw is put in place eccentrically with respect to the hole, as is necessary for attaining compression of a fracture. Additionally, the non-threaded portion of the hole may flare outward in the area of the bone contacting surface to provide for increased angulation of the bone screw with respect to the bone plate.
The threaded portion of the hole may be positioned closer to the end of the elongated hole that is closer to the center of the plate, thus avoiding any undesirable effects on the compression capability of the non-threaded portion. Thus, when the bone plate is used as a compression plate, the geometry of the non-threaded portion (compression portion) is not adversely affected by the presence of the threaded portion.
As discussed above, the hole may be elongated. Thus, the elongated hole may define first and second dimensions on the bone contacting surface, wherein the first dimension DL is substantially parallel to the longitudinal axis of the bone plate and the second dimension DQ is substantially perpendicular to the longitudinal axis. The ratio of DL/DQ may be within the range of about 1.1 to 3, and preferably is in the range of about 1.1 to 1.5. This ratio follows from the combination of the threaded portion (locking portion) with the non-threaded portion (compression portion), which requires a clamping path for the screw head. This ration of DL/DQ represents a preferred compromise between compression and plate weakening due to the presence of the hole.
According to another embodiment of the invention, the underside of the bone plate may be concave, thus allowing the plate to conform to the rounded cross-section of the tibia, femur, humerus, lower arm bone, and other bones with which the present invention may be used. The concave configuration of the underside also allows a conventional bone screw to be inserted obliquely through the plate hole. This feature may be especially important when gripping a small bone fragment, which must be pulled up to the plate.
The present invention is also directed to a bone plating system including at least one bone screw having a screw head with a thread disposed thereon, the thread being configured and dimensioned to engage the threaded portion of the above-described bone plate holes. Preferably, the bone screw is self-tapping and/or self-drilling.